This invention relates in general to the placement of electronic components on printed circuit boards and, more specifically to a method for very precisely placing high pin count, fine pitch, components on a circuit board for soldering thereto.
Originally, printed circuit boards included plated holes into which leads of electronic components were inserted and soldered. Currently, circuit boards tend to have closely spaced surface pads with the component simply placed on the board with leads in contact with the pads, to which the leads are then soldered.
The pattern of closely spaced wires and pads for connection to component leads are usually formed on the printed circuit board base by silkscreening techniques. After board fabrication and during assembly operations, a stencil having through holes conforming to the pad pattern is placed over the pad array and precisely aligned therewith. A solder paste made up of very small solder balls in a slurry of flux and other ingredients is wiped over the stencil with a squeegee, forcing small amounts of paste through the holes onto the pads. The stencil is lifted away leaving the paste on the pads. The components must then be very precisely placed on the board with the component leads aligned with the pads. Accuracies on the order of 0.0005 inch are often required. Once the component is placed, the solder is melted, generally in an infrared or convection oven, and bonds between the pad and lead.
Very large, very expensive, robotic machines have been developed for accurately placing components on such surface mount boards with leads precisely aligned and soldering the leads to the pads in very high volume manufacturing operations. These machines make use of extremely expensive vision alignment equipment and other optical devices to precisely locate the components. These large and complex machines require considerable operator training and the component and board designs cannot be rapidly and easily changed. Exemplary of such machines is that described by Takahashi et al. in U.S. Pat. No. 4,292,116. Such large devices are not economically feasible, or well adapted to, low to medium volume operations where only a few fine pitch parts are placed on relatively few boards.
Attempts have been made to develop flexible, rapidly adaptable, devices that can be set up to accurately place different surface mount components on varying boards. Two basic methods are used to perform the component to pad pattern orientation, the "tooling pin" method and the "pad location" method.
The tooling pin method takes advantage of tooling holes formed in the board. The tooling holes are precisely oriented relative to the pad pattern. A stencil is formed with openings conforming to the pad pattern along with tooling holes and oriented relative to the tooling pins in the same position as the printed circuit board. Registration pins are placed in the tooling holes and a component is placed with leads extending through the pad pattern openings. The component is then lifted vertically and a "printed" board (that is, a board with solder paste on the pads) is placed on registration pins with the component then lowered vertically, placing the leads back onto the printed pads. The solder paste is then heated to solder the leads to the pads.
while effective where great precision is not required, this method suffers from excessive tooling hole tolerances. Typically, tooling holes have and "X" and "Y" tolerances of about 0.005 inch, with a true position radius tolerance of about 0.007 inch. The resulting error is unacceptable with any component with leads below a 0.020 inch distance or "pitch" between two adjacent leads. The resulting error is unacceptable with any component with leads below a 0.020 inch distance or "pitch" between two adjacent leads. Printed circuit boards manufactured in the same production "lot" can easily achieve tolerances of about 0.005 inch with a true position radius of about 0.0007 inch. Since the tooling holes in the stencil are fixed, it cannot take advantage of the variations in tooling hole locations on a given lot of boards. Typical of devices using an index pin and hole orientation technique are those described by Wasserman in U.S. Pat. No. 4,595,794 and by Conroy in U.S. Pat. No. 4,985,107.
Others have attempted to use an orientation fixture engaging the corners or edges of the component to align the component leads with the board pads. Typical of these is the device described by Read in U.S. Pat. No. 4,722,135. Since the tolerance of corners and edges are generally not sufficiently tight, complex measuring and adjustable alignment mechanisms are generally required.
Thus, there is a continuing need for methods of placing fine pitch surface-mount electronic components onto printed surface with very high accuracy with simple equipment requiring limited training of operators and capable of rapidly changing to different boards and components for short production run work.